Vasopressin (AVP), the antidiuretic hormone is a peptide hormone actively involved in the regulation of body fluid osmolality, blood volume, blood pressure, and cell proliferation via the stimulation of specific membrane- bound receptors classified into V1a-vascular, V2-renal, and V3-pituitary subtypes having distinct pharmacological profiles and intracellular second messengers. The secretion of AVP is an absolute requirement for the maintenance of fluid homeostasis as shown in human and experimental models of diabetes insipidus. The usefulness of natriuretic agents has been demonstrated in conditions associated with water and salt retention. However, there are diseases (hyponatremia of congestive heart failure, liver cirrhosis, nephrotic syndrome, and the syndrome of inappropriate secretion of antidiuretic hormone, SIADH) due to the inappropriate release of vasopressin and characterized by an excess retention of water. An agent that increases urine output by producing a free water diuresis would be of significant advantage in these conditions. This study is designed to evaluate the efficacy, safety, and tolerance of a single oral dose of VPA-985 in patients with hyponatremia due to SIADH. In addition, preliminary pharmacokinetic and pharmacodynamic parameters will be assessed. This study has just begun patient recruitment. Up to 20 patients with diagnosed SIADH between the ages of 18 and 75 years will be enrolled.